Catalyst materials promote chemical reactions but do not themselves enter into the output product nor are they consumed by the reaction. “Heterogeneous catalysis” refers to a catalytic process in which the physical states of the catalyst and reactant (material involved in the chemical reaction) are different. This is distinguished from “homogeneous catalysis”, where the reactant and the catalyst have the same physical state and, as a result form solutions or miscible mixtures (liquid/liquid; gas/gas). For example, the physical state of heterogeneous catalyst material may typically be solid-phase (e.g., a metal or ceramic) while the reactants may be gases and/or liquids. Therefore, the “surfaces” of a solid catalyst material that may contact a reactant play a significant role in catalysis.
However, with advancing knowledge of the nature of “states of matter” many conventional theoretical models of solid, liquid or gas may be poorly suited to describe the range of states of matter. A “surface” displays far more complexity than an oversimplified image of a visual plane used in many conventional models to describe it. The theoretical model of a sharp boundary as characteristic of a surface may be misleading for understanding certain catalytic surface activity. Instead, a surface of a solid may be viewed as a zone or transition region where close spaced atomic groups inside the solid taper off as the view looks toward the edge of the surface zone. Inside, the solid components are closely bound but at the surface such bonding is disturbed.
For more than a century, countless specific examples of catalysts have been cataloged, developed and applied. The many known reactions presently form the foundation of most of the world's chemical industry. Recognition of catalytic effects began at the dawn of the 19th century. In the early 20th century, many large industrial scale reactions began utilizing important industrial processes using heterogeneous catalysts. Notable examples are the Haber-Bosch ammonia synthesis (fertilizers for world agriculture), the Fischer-Tropsch hydrocarbon synthesis (oil, gasoline and hydrocarbon materials), and the synthesis of plastic materials, resulting in a vast polymer chemical industry. Catalytic processes in the chemical industries of the world currently have enormous commercial significance. A large fraction of all chemical production is catalyst based. Some approaches to fundamental theories of catalysis exist, such as the Density Functional Theory, which involves mathematical approximation of some quantum-mechanical factors representing chemical bonding. Nevertheless, development of products and processes remain largely based on pragmatic experimental approaches. Consequently, the field of heterogeneous catalysis is replete with “recipes” for producing various kinds of catalysts utilizing a wide variety of materials and constructions. In fact, catalysts are often known by the molecular species of their reactions rather than by their mode of action or even by their construction. Three typical examples of recent US Patents involving catalysts are: U.S. Pat. No. 6,821,922 to Tacke et al “Supported Catalyst For The Production Of Vinyl Acetate Monomer”; U.S. Pat. No. 6,852,669 to Voit et al “Hydrogenation Catalyst”; and U.S. Pat. No. 6,867,166 to Yang et al “Selective Adsorption Of Alkenes Using Supported Metal Compounds”. A product descriptive brochure from the Johnson Matthey Company, a major supplier of catalyst materials, is similarly functionally descriptive for each of a group of Palladium based products the company offers for carbon-carbon bonding. (See brochure available on the Internet at www.amcpmc.com/pdfs/producttype/45.pdf). In short, the chemist's understanding of the foundational fundamentals of catalyst art progresses but as yet is incomplete.
Heterogeneous catalysts having a spherical particle shape have often been employed as catalysts and catalyst substrates. Such interest has typically been in pursuit of large apparent surface area for contact with reactant, with some added concern for thermal properties such as heat transfer in exothermic reactions. For example, U.S. Pat. No. 6,747,180 to Ostgard et al, “Metal Catalyst” describes the forming of hollow metallic spheres of 0.5 mm to 20 mm diameter. Its focus appears to be reduction of the amount of expensive metal unavailable in the sphere's interior to the catalytic surface of the desired spherically shaped particles.
U.S. Pat. No. 5,237,019 to Weiland et al, describes small spherical particles of from 0.01 to 3.0 mm diameter composed of organosiloxane materials containing platinum group metals. The particles are specified to have a bulk density below that of water while allowing a wide range of surface area to be obtained from varied particle size. Obtaining large surface area this way appears a major objective. Emphasis is also placed on the character of the catalyst metal dispersed in of such compositions.
U.S. Pat. No. 6,518,220 to Walsdorff et al, describes “Shaped Catalysts” of a hollow cylindrical or annular form of a catalytically active material. Improved selectivity of the preferred shape as well as reduced pressure drop are the disclosed objectives of the design.
In several U.S. Patents to Wang et al (U.S. Pat. Nos. 4,804,796, 4,701,436 and 4,576,926), hollow spheres are disclosed that are formed in various ways to enable the effective density of such spheres to be made to allow such to float in a medium of choice. An object of these patents is to improve the dispersion of such catalyst in the selected reactant medium.
U.S. Pat. No. 3,966,644 to Gustafson, titled “Shaped Catalyst Particles” describes a longitudinally symmetric trilobe shaped alumina composite catalyst particle having a narrow range of sizes and specific porosity characteristics claimed useful for hydrocarbon conversions of petroleum residuum. The shape is discussed in terms of its void ratio and flow properties, improved activity, claimed longer duration of effective operating time and, superior crush resistance.
U.S. Patent Application US 2005/0130837 by Hoek et al, titled “Shaped Catalyst Particles For Hydrocarbon Synthesis” describes a trilobular extruded shaped catalyst form, having a void ratio in excess of 50%, well in excess of the 43% or so of other trilobal designs. Flow rates appear to be a principal concern of these applicants.
U.S. Pat. No. 4,293,445 to Shimizu et al “Method For Production of Molded Product Containing Titanium Oxide” discloses the addition a small proportion of barium for improving the strength of the ceramic catalyst product.
A focus of conventional improvements in the catalyst art has been to maximize the surface area of catalyst material exposed to the reactant. This has been accomplished via various means: in one way through creation of powdery and porous materials; in another through high surface area geometries; in another through the use of chemical processes acting on the catalyst's surface to “activate” or “refresh” it.
Certain investigators outside the field of chemistry and catalysis have observed what they believed to be detrimental effects of surface-surface contact in the context of electrical switches and relays. Such phenomena were studied in a series of papers coming from The Bell Laboratories in the early 1950s (See, The Bell System Technical Journal, May 1958 pp 738-776, “Organic Deposits on Precious Metal Contacts” By H. W. Hermance & T. F. Egan). The Bell Labs workers' motivation for the study came from examining the intermittent failure of telephone exchange switching relays caused by accumulation of organic deposits formed on their contacts.
Surprisingly this problem was exacerbated when efforts were made to hermetically seal a very large number of switching relays employed in a telephone exchange of that era. The sealing effort initially seemed desirable to protect the contacts from dust and airborne contaminants. However, small amounts of organic vapor inside the sealed relays (coming from magnet wire, insulation, and other organic material of their fabrication) were not eliminated and deposited on the contacts sealed inside. The resulting problems were severe because the “open” circuit caused by a deposit would soon disappear, making it difficult to locate. The Bell researchers devised non-current carrying relay-contact-operating mechanisms to evaluate various kinds of contact material and environments. The signal circuits that appeared most vulnerable carried essentially no current through the relay contacts and operated only with very small signal voltages. Such “dry circuit” operation could provide no arcing actions that might clean contacts. The Bell Labs researchers discovered that the carefully chosen corrosion resistant group 10 (platinum group) contact metals were very prone to forming the disturbing organic deposits they named “contact polymers.”
While much effort has been directed to increasing catalysts' effective surface area as determined by gas adsorption tests, the resulting increase in surface complexity and porosity has also led to detrimental reactant trapping and retarded movement of reaction materials. Accordingly, improved heterogeneous catalysts, catalyst systems and catalytic reaction methods are still needed.